Re: [PATCH] userfaultfd: mark uffd_wp regardless of VM_WRITE flag

On Feb 17, 2022, at 5:58 PM, Peter Xu peterx@redhat.com wrote:

Hello, Nadav,

On Thu, Feb 17, 2022 at 09:16:02PM +0000, Nadav Amit wrote:

...

From: Nadav Amit namit@vmware.com

When a PTE is set by UFFD operations such as UFFDIO_COPY, the PTE is currently only marked as write-protected if the VMA has VM_WRITE flag set. This seems incorrect or at least would be unexpected by the users.

Consider the following sequence of operations that are being performed on a certain page:

mprotect(PROT_READ) UFFDIO_COPY(UFFDIO_COPY_MODE_WP) mprotect(PROT_READ|PROT_WRITE)

No objection to the patch, however I'm wondering why this is a valid use case because mprotect seems to be conflict with uffd, because AFAICT mprotect(PROT_READ|PROT_WRITE) can already grant write bit.

In change_pte_range():

   if (dirty_accountable && pte_dirty(ptent) &&
                   (pte_soft_dirty(ptent) ||
                           !(vma->vm_flags & VM_SOFTDIRTY))) {
           ptent = pte_mkwrite(ptent);
   }

I think you are right, and an additional patch is needed to prevent mprotect() from making an entry writable if the PTE has _PAGE_UFFD_WP set and uffd_wp_resolve was not provided. I missed that.

I’ll post another patch for this one.

PS: I always think here the VM_SOFTDIRTY check is wrong, IMHO it should be:

   if (dirty_accountable && pte_dirty(ptent) &&
                   (pte_soft_dirty(ptent) ||
                   (vma->vm_flags & VM_SOFTDIRTY))) {
           ptent = pte_mkwrite(ptent);
   }

Because when VM_SOFTDIRTY is cleared it means soft dirty enabled. I wanted to post a patch but I never yet.

Seems that you are right. Yet, having this wrong code around for some time raises the concern whether something will break. By the soft-dirty I saw so far, it seems that it is not commonly used.

Could I ask why you need mprotect() with uffd?

Sure. I think I mentioned it before, that I want to use userfaultfd for other processes [1], by having one monitor UFFD for multiple processes that handles their swap/prefetch activities based on custom policies.

I try to set the least amount of constraints on what these processes might do, and mprotect() is something they are allowed to do.

I would hopefully send the patches that are required for all of that and open source my code soon. In the meanwhile I try to upstream the least controversial parts.

[1] https://lore.kernel.org/linux-mm/YWZCClDorCCM7KMG@t490s/t/